Modems
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Modems. Chapter 17. Modems are little devices to use the telephone to talk to other computers. Modem is an abbreviation for Modulator/ Demodulator Modems can be internal or external. Basic Knowledge. Modem Signals.

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Modems

Modems

Chapter 17


Basic knowledge

  • Modems are little devices to use the telephone to talk to other computers.

  • Modem is an abbreviation for Modulator/ Demodulator

  • Modems can be internal or external.

Basic Knowledge


Modem signals

Modem Signals

  • Modems take analog signals and convert them into digital signals and vice versa.

  • The data that goes into the modem is called serial communication and is a series of 0’s and 1’s. This data is converted into bytes via the UART(Universal Asynchronous Receiver/Transmitter) so CPU’s can understand information

  • There are 2 ways of “chopping up” the incoming signals

    • Asynchronous

    • Synchronous


Asynchronous organization

Asynchronous Organization

  • The main way to serial ports communicate

  • The data is “chopped up” into 7 or 8 bit packets. 8-bit is most common

  • Each Packet starts with a start bit that tells the receiving modem that it is the beginning of a piece of information and it always is zero and ends with a stop bit that tells the receiver the packet is over.

  • Asynchronous communication packets have an optional parity bit used for error detection. Parity bits are rare

  • Asynchronous packet settings are described in a particular order: data bits, parity type, then stop bits.

    • Example: If a system uses 8 data bits, no parity, and 1 stop bit, you’d say 8-N-1.


Synchronous organization

Synchronous Organization

  • Mainly for uploading and downloading programs to and from your computer.

  • Protocols are sets of definitions used by communication programs so that both computers are speaking the same language. There are many protocols that basically work the same way.

  • When a modem is going to send data with synchronous communication it sends a series of standardized bytes called sync bytes, which tell the receiving port that it is about to get information.

  • After sync bytes the sending modem adds a start-of-text(STX) marker then sends the data.

  • After data transmission the packet ends with an end-of-text(ETX) marker and error checking characters(ECC)

  • The receiver responds with an acknowledge if data is good or a no acknowledge if there is an error.


Ftp s and baud rate

FTP’s and Baud Rate

  • File Transfer Protocol

    • FTP is used to enable you to exchange files with mainframes, mini’s, or other PC’s. It should be included in the software package that comes with your modem

    • Refer to pages 1046-1047 for variations of FTP.

  • Baud Rate

    • Baud rate is the time that a modem uses as its carrier frequency

    • Baud rate speed is measured in bits per second

      • Example: If baud rate was 2400 and modulated at 2 bits per cycle the bps would be 4800


Flow control

Flow control

  • Flow Control (Handshaking)

    • Process by which 2 serial devices verify a conversation.

    • During data transfer there are 2 conversations when flow control is needed, Local(between modem and com port) and end-to-end(between modems).

    • Flow control can come in hardware and software. Look at pg. 1049 to learn about each.


Interface signals

Interface Signals

  • CD – Carrier signal detected

  • RD or RX – Receive Data

  • TD or TDX – Transmit Data

  • DTR – Data Terminal Ready

  • DSR – Data Set Ready

  • RTS – Request To Send

  • CTS – Clear To Send


Modem commands

Modem Commands

  • Commands of modems are known as the command set.

  • There is no standard for modem commands, but there is a main set that most companies follow. It is the famous AT command set.

  • AT commands are entered through the keyboard.

  • A list of AT commands is on pg. 1052-1053


Modem commands1

Modem Commands

  • AT – checks to see that modem is plugged in

  • ATH – Hangs up

  • ATM1 – turns the speaker on

  • ATM0 – turns the speaker off

  • ATD – dials a number

  • ATZ – resets the modem


Talking to serial ports and telephone lines

Talking to serial ports and telephone lines

  • Telephone Lines

    • A phone line is guaranteed to handle speeds only up to 2400 baud

    • To verify you are getting the best quality is to call the phone line.

    • If the company thinks the voltage drop is too much, they will condition the line. Conditioning is adding a little voltage to the line to compensate for the voltage drop.

  • Talking to serial ports

    • Data communication equipment is the device that sends data. Data Terminal Equipment is the device that receives data. The connection between the 2 is called the handshake.

    • Chart on page 1054


Uart s

UART’s

  • Heart of many serial ports.

  • It takes parallel information from data buses and makes it serial information to be sent out to a modem.

  • It’s important to know what type of UART is in your COM ports. You can find out the type by checking the diagnostics tab/ More Info button under the Modems applet in the control panel under windows 9x. In Windows 2000 use the Phone and Modem Options applet.

  • For UART’s check pg. 1056


Fax modems

Fax Modems

  • Fax technology was invented in 1842 by Alexander Bain

  • Modems send data in single dots which are converted to text.

  • 4 different classes or groups of fax standards

    • Groups 1 and 2 were analog standards for fax modems operating at 300 baud.

    • Group 3 was standardized for digital facsimile devices to communicate over phone lines. Fax transmission had a max speed of 14400 bps.

    • Within group 3, there are several different classes that explain devices using different speeds and methods of communication.

    • Any good fax modem software can run G3

    • Chart of Fax standards on 1058


Modem negotiations

Modem Negotiations

  • Standardized series of responses between two modems so they can determine optimal speed, data compression, error detection, etc.

  • Chart pg. 1059-1060


Modems

TAPI

  • TAPI is a set of dynamic link libraries (DLL) that enables Windows 9x to perform telephone functions.

  • A good TAPI installation will configure the modem type, I/O address and IRQ, local phone number, long distance dialing codes, and etc. to enable it to use the modem.

  • TAPI is also used for answering machines, handle faxing, call forwarding, caller ID, and paging


Installation and troubleshooting

Installation and Troubleshooting

  • Modems rarely fail, but problems usually are COM port problems

  • Set up and verify the com ports

    • Make sure you install nonconflicting I/O addresses and interrupts.

    • Check modems in device manager to verify the modem is working properly.

    • Windows 9x and 2000 PnP technology makes installation very easy. To do it manually use the add new hardware wizard.

  • Verify the Modem’s BIOS

    • Modern modems all have on board BIOS

    • A better term for BIOS would be command set

    • The BIOS is stored on the flash ROM of a modem.

    • BIOS can be corrupted easy by simply removing and reinserting the modem


Set up the correct command set

Set Up the Correct Command Set

  • All that Windows communication programs need to handle a modem are the latest IFN files for your modem and version of Windows.

  • If Windows doesn’t verify the modem try a different brand.

  • Windows 2000

    • For dial configuration use the phone and modem option and the control panel

    • Windows 2000 will not detect some internal modems so you must do manual installation.


  • When modems break

    When Modems Break

    • Check COM port and IRQ port

      • Program called modem doctor is very good for this

      • Modem doctor verifies COM ports and tests your modem and tells what kind of shape its in.

    • If your software says there is no modem

      • Make sure modem is looking at correct port

      • Check that there is no conflict with other devices

      • Reinstall all software and reconfigure your modem.

    • Modem works sporadically

      • Make sure you have the right modem installed.

      • Try different type.

      • Check phone lines in house.


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